The Musical Box has an interview with folk/jazz singer Sonya Kitchell where she mentions touring with Herbie Hancock in support of his Grammy-winning album River: The Joni Letters, and meeting Joni Mitchell.
“I got to hang out with her for a couple of days, which was really lovely,” Kitchell said. “She is a great lady. She heard me singing her songs and dug it. That was very cool. So when she asked me, ‘Do you want to sing harmonies with me on one of my new songs?’ I said, ‘Well… yes.’”
The song turned out to be “Hana” from Joni Mitchell’s latest album Shine.